docs: def restricted_to_files(self, filenames: List[str]) -> 'FileLineSet':
          """
          Returns a variant of this set that only contains lines that occur in
          any one of the given files. (I.e., returns the intersection of this set
          and the set of all lines from a given set of files.)
          """
          restricted = {} # type: Dict[str, Set[int]]
          for fn in filenames:
              if fn in self.__contents:
                  restricted[fn] = set(self.__contents[fn])
          return FileLineSet(restricted)